The present invention relates to hybrid switching devices employing solid-state crowbar protection circuitry and, more particularly, to such hybrid electrical switches in which a mechanical current switching element employs one or more liquid metal contacts.
Electromechanical switches employed in the interruption and initiation of electrical current flow paths generally have certain problems associated with them. When such current-carrying switches are removed from their closed to their open positions, inherent inductive effects amost always result in some arcing between the electrical contacts. This arcing can erode and degrade the contacts so that, eventually, the electromechanical switch no longer operates in an acceptable fashion. The principle function of these separate electromechanical contacts is to withstand circuit voltages when the contacts are separated and to assure low contactor impedance when they are closed. The contact resistance of the separable contacts is a critical variable applicable to the closed position. Stability of this contact resistance is often affected by chemical reaction between the contacts and the ambient atmosphere, espcially during arcing. Furthermore, for composite contact materials, segregation of materials near the contact surface with repeated melting and solidification caused by arcing also adversely affects the contact resistance. Moreover, the electromechanical device must be designed so that significant mechanical pressure is applied to the contacts in the closed position to provide the low contact resistance desired. For proper operation, the mechanism must maintain enough contact force to provide microdeformations in the contacts to increase the effective area of the contact. In general, the contact resistance is proportional to the resistivity of the material employed, directly proportional to the square root of the hardness of the material and inversely proportional to the square root of the contact pressure. Accordingly, it is seen that contact material properties such as hardness and resistivity play a significant role in determining contact resistance in electromechanical switches. Furthermore, it is also highly desirable that such materials exhibit low oxidation rates since switches are often exposed to atmospheric conditions. If oxidation of the contact material does occur to any significant degree, it is furthermore necessary that any of the oxidation products not significantly interfere with the functioning of the contacts. These criteria have generally limited the viable contact materials to such substances as silver and gold, alloys thereof, and/or materials employing significant amounts of these expensive materials.
In order to mitigate arcing effects in particular, many switching devices have been proposed in which electromechanical contacts are connected in parallel with a solid-state switching device, such as a thyristor or silicon-controlled rectifier. The solid-state switching element is used to divert or "crowbar" the current away from the contacts either just before or just after contact separation, thereby reducing arcing and easing the requirements of the electromechanical device and its contact materials. Crowbar circuits are described by Horowitz and Hill in their text "The Art of Electronics", Cambridge University Press, 1980, on pages 176-177 thereof. The solid-state switching element is also used to initiate current flow through the switching device just prior to closing the solid contacts, thereby lowering the device voltage and reducing the probability of arcing between the contacts. However, studies conducted by others have indicated that even with the use of additional crowbar circuitry, solid contacts for hybrid circuit switching devices are still likely to involve silver-base materials, at least as an overlay material on another base material such as copper. This conclusion is based primarly on the ability of silver to provide and maintain low contact resistance due to its low electrical resistivity, medium to low hardness, and its relative inertness to surface-contaminating chemical reactions.
A large number of individuals skilled in the art of designing electrical circuit switching devices have proposed the use of solid-state circuitry in parallel with electro-mechanical contacts to reduce arcing on these contacts when opening or closing under load or fault conditions. The following list of patents all appear to disclose such crowbar circuitry in conjunction with electromechanical switching devices. However, all of the patents listed below further appear to involve the use of solid metal contacts employing an essentially standard design. This list includes the following patents: U.S. Pat. No. 2,058,808, issued Nov. 1, 1960 to W. Miller--"Electrical Arc Suppressor"; U.S. Pat. No. 3,237,030, issued Feb. 22, 1966 to R. J. Coburn--"Radio Noise-Free Switch"; U.S. Pat. No. 3,321,668, issued May 23, 1967 to E. S. Baker--"Current Control Apparatus"; U.S. Pat. No. 3,330,992, issued July 11, 1967 to A. R. Perrins--"Electric Switch"; U.S. Pat. No. 3,339,110, issued Aug. 29, 1967 to J. P. Jones--"Relay Circuits"; U.S. Pat. No. 3,389,301, issued June 18, 1968 to E. I. Siwko--"Arc Suppressing Circuit"; U.S. Pat. No. 3,395,316, issued June 30, 1968 to P. A. Denes et al.--"Electrical Switch With Contact Protector"; U.S. Pat. No. 3,402,302, issed Sept. 17, 1968 to E. J. Coburn--"Radio Noise-Free Switch"; U.S. Pat. No. 3,466,503, issued Sept. 9, 1969 to L. F. Goldberg--"Assisted Arc AC Circuit Interruption"; U.S. Pat. No. 3,474,293, issued Oct. 21, 1969 to E. I. Siwko et al.--"Arc Suppressing Circuits"; U.S. Pat. No. 3,504,233, issued Mar. 31, 1970 to E. L. Hurtle--"Electric Circuit Interrupting Device With Solid State Shorting Means"; U.S. Pat. No. 3,539,775, issued Nov. 10, 1970 to C. F. Casson--"Double-Make Contact Switching Apparatus With Improved AC Arc Suppression Means"; U.S. Pat. No. 3,555,353, issued Jan. 12, 1971 to C. F. Casson--"Means Effecting Relay Contact Arc Suppression in Relay Controlled Alternating Load Circuits"; U.S. Pat. No. 3,558,910, issued Jan. 26, 1971 to R. G. Dale et al.--"Relay Circuits Employing a Triac to Prevent Arcing"; U.S. Pat. No. 3,588,605, issued June 28, 1971 to C. F. Casson--"Alternating Current Switching Apparatus With Improved Electrical Contact Protection"; U.S. Pat. No. 3,614,464, issued Oct. 19, 1971 to W. V. Chumakov--"Arcless Tap- or Source-Switching Apparatus Using Series-Connected Semiconductors"; --U.S. Pat. No. 3,633,069, issued Jan. 4, 1972 to G. Bernard--"Alternating Current Circuit-Interrupting System Comprising a Rectifier Shunting Path"; --U.S. Pat. No. 3,639,808, issed Feb. 1, 1972 to G. R. Ritzow, "Relay Contact Protecting Circuit"; U.S. Pat. No. 3,783,305, issued Jan 1, 1974 to P. Lefferts--"Arc Elimination Circuit"; U.S. Pat. No. 3,982,137, issued Sept. 21, 1976 to J. K. Penrod--"Arc Suppressor Circuit"; U.S. Pat. No. 4,025,820, issued May 24, 1977 to J. K. Penrod--"Contactor Device Including Arc Suppression Means"; U.S. Pat. No. 4,074,333, issued Feb. 14, 1978 to K. Kurakami et al.--"AC Relay System"; U.S. Pat. No. 4,152,634, issued May 1, 1979 to J. K. Penrod--"Power Contactor and Control Circuit"; U.S. Pat. No. 4,068,273, issued Jan. 10, 1978 to A. Metzler--"Hybrid Power Switch".
However, it is apparent that hybrid switching devices must, of necessity, incur an added cost associated with the circuitry for performing the crowbar function. Accordingly, for greater cost competitiveness with conventional electromechanical switches, it is highly desirable that the cost of the solid-state crowbar circuitry be compensated by changes in the design of the electromechanical portion of the hybrid switching device. In particular, two of the large cost elements associated with most low voltage (less than 1,500 volts) switches are the contact material and the driving mechanism. The contact material is expensive because it preferably employs a noble metal such as silver or gold. The drive mechanism also tends to be expensive in that it requires mechanical devices for holding the switch contacts in a forcibly closed position with sufficient pressure to cause microdeformations and yet, in the next instant of time, to quickly separate the contacts.
In sum, it is seen that electromechanical switching devices generally require the use of relatively expensive contact material. Furthermore, it is seen that even in situations employing crowbar circuitry to mitigate arcing effects, expensive contact material is also generally required. Furthermore, it is seen that the cost of the crowbar circuitry has in the past added significantly to the cost of hybrid switching circuit devices without concomitant savings associated with the electromechanical portion of the switch. It is further seen that while liquid metal contact switching devices have been employed in the past, they have generally not been employed in circuits in which high arcing currents are a consideration. This is generally the result of vapor pressure problems associated with liquid metal contact devices. In liquid metal switches which are opened to the atmosphere, arcing can rapidly contribute to vaporization of the liquid metal. In the case that the liquid metal is mercury, it is generally appreciated that the escape of mercury vapor to the surrounding atmospheric environment would generally be detrimental. In the situation in which the liquid metal is contained within a sealed environment, normal arcing in the switch results in the build up of significant vapor pressures from the volatilized liquid metal. Containment of these high vapor pressures is a significant design challenge, solved only at added cost to the device. Accordingly, for these reasons it is seen, particularly from the list of patents cited above, that the use of liquid metals in switches carrying high levels of arcing current has not been employed.